1. Field of the Invention
The present invention relates to an image reading apparatus that uses an image device to read an original document.
2. Description of the Related Art
Conventionally, for image reading by copying machines, scanners, etc., image reading apparatuses using light-emitting devices and image devices have been used. An image reading apparatus of the related art is described below with reference to FIGS. 13 and 14.
FIG. 13 is a perspective view of an image reading apparatus 1 of the related art, and FIG. 14 is a detailed view of the main part of the image reading apparatus 1. The image reading apparatus 1 has, on its top, a contact glass 2 on which an original document is set. The image reading apparatus 1 includes a frame member 3 that slides when the original document is read. The frame member 3 has an image capturing unit 31 on its top. The image capturing unit 31 includes, in its longitudinal direction, an image device 32 made of a contact image sensor (CIS) or a charge-coupled device (CCD), and a light-emitting device 33 which is made of a halogen lamp and which is integrated with the image device 32. The frame member 3 has, on its bottom, an image-capturing printed circuit board 4 for driving the image capturing unit 31. The image capturing unit 31 is electrically connected to the image-capturing printed circuit board 4, and is driven by a signal from the image-capturing printed circuit board 4. The frame member 3 is supported by supporters 9a and 9b so as to slide along the supporters 9a and 9b, and is slid by a motor (not shown). A controlling printed circuit board 5 is fixed to a bottom portion of the image reading apparatus 1, and controls the sliding operation of the frame member 3, and the image reading operation and light-emitting operation of the image capturing unit 31. The image-capturing printed circuit board 4 and the controlling printed circuit board 5 are connected to each other by a cable wire 10. Image signals based on the original document read by the image capturing unit 31, and control signals for the image reading operation and the light emitting operation are transmitted and received via the cable wire 10, as needed.
The image-capturing printed circuit board 4 has a connector 21 at an end thereof, and the controlling printed circuit board 5 has a connector 22 at an end thereof. The ends of the cable wire 10 are connected to the connectors 21 and 22. The cable wire 10 is flexible and can be transformed and moved following the sliding operation of the frame member 3.
In addition to the cable wire 10, a power-supply cable wire 11 is connected to the image-capturing printed circuit board 4. The power-supply cable wire 11 is used to supply power for the sliding operation of the frame member 3 and for the image reading operation and the light emitting operation of the image capturing unit 31. The power-supply cable wire 11 is connected to a power-supply board (not shown). The power-supply cable wire 11 is flexible similarly to the cable wire 10 and can be transformed and moved following the sliding operation of the frame member 3.
Driving signals that drive the light-emitting device 33 and the image capturing unit 31 for image reading require a large amount of current in order to perform photoelectric conversion. Also, to realize high speed reading, a high frequency signal is used. Accordingly, whenever an image is read, transmission and reception of signals having high frequency and large current is repeatedly performed between each of the image device 32 and the light-emitting device 33, and an integrated circuit for driving both. Thus, emission is generated, which is a big problem.
Through the power-supply cable wire 11, signals for driving the image device 32 and the light-emitting device 33 are transmitted as so-called “common mode currents”, using a power-supply pattern and a ground pattern on the printed circuit board 4 as paths. This is a similar big problem of emission.
Regarding preventive measures for the emission from the cable wire 10 and the power-supply cable wire 11, methods are conventionally known which prevent the generated emission by incorporating a ferrite core and by accommodating in a shielded box housing the whole cable wire and, if necessary, a printed circuit board.
Nevertheless, in recent years, the amount of emission has been increasing due to the higher frequencies of driving signals and image-information signals which are set for meeting the demand of high speed image processing. Accordingly, a plurality of frequency bands have a large amount of emission, so that it is difficult to prevent the emission by using filter components such as ferrite cores. In addition, the ferrite cores are expensive, and the number of usable ferrite cores is limited.
Also, with an increased number of driving signals and an enlarged circuit size, there are many cases in which the area of a printed circuit board itself must be increased. Accordingly, to use a shielded box to shield the emission, its size is an issue. When the shielded box is further enlarged, there is a possibility that emission caused by cavity resonance depending on the box size may be generated in a frequency range of 30 MHz to 1 GHz in which some measures should be taken. In frequencies generated due to the cavity resonance, currents having corresponding frequency components are repeatedly reflected by a shield conductor and are mutually superimposed, whereby the currents exhibit distributions that are not stationary canceled. As a result, the effect of the shield is damaged, thus causing a high intensity of emission. The frequency generated by the cavity resonance is dependent on the size of a shield. Thus, to suppress the cavity resonance, by forming a partition structure in the box so that the box size is virtually reduced, the generated emission frequency can be excluded from frequencies observed for emission measurement. In this way, it is possible to change the generated emission frequency to a high frequency, but the shield structure is complex and assembly cost increases.
According to Japanese Patent Laid-Open No. 11-187223, when the disclosure is described by using the above reference numerals, the driving signals are generated by the image-capturing printed circuit board 4 connected to the image capturing unit 31, and by using a trigger signal, the image-capturing printed circuit board 4 and the controlling printed circuit board 5 are synchronized with each other. This makes it possible to limit signals output from the controlling printed circuit board 5 to the image-capturing printed circuit board 4 to only the trigger signal. Thus, the number of signal lines in the cable wire 10 can be reduced and the emission can be suppressed.
Nevertheless, since the cable wire 10 must be provided with a ground wire, a high frequency current is transmitted by the ground line of the cable wire 10, so that a similar problem of emission occurs. Also, since a read image must be set to have a higher resolution in order to realize high-resolution image processing, the number of signal lines constituting the cable wire 10 tends to increase with an increase in the number of image signals. Accordingly, there is a limitation in reducing the number of signal lines in the cable wire 10. For a high frequency current causing the emission, transmission paths increase, and it becomes difficult to take preventive measures for the emission.